1. Field
The present disclosure relates to nanoparticles and methods of manufacturing the same, and more particularly, to nanoparticles comprising an inorganic capping agent and methods of manufacturing the same.
2. Description of the Related Art
A quantum dot (sometimes abbreviated as “QD” throughout the application) or a nanocrystal is a crystal structure of a dimension from a few to several ten nanometers, of which properties vary depending on its size. A semiconductor QD demonstrates properties between those of bulk semiconductors of the same material and those of discontinuous molecules. Since its physical, chemical, and electrical properties may be controlled by changing the size of the same material due to quantum confinement effects and large surface to volume ratios, a nanoparticle has received great interest for a new method of controlling properties of a material and a new material.
A typical nanoparticle, of which dimension varies from several to several hundred nanometers, may be comprised of two or three main parts: the middle core part which is a single crystal, of which the diameter may be about one to several ten nanometers, a shell or multiple shells, and a coating region or a coating layer of ligands formed on a surface of the core or the shell. Among preparation methods of nanoparticles, a wet-chemical synthesis method, which makes colloidal nanoparticle core, may produce a mass amount of uniformly nanosized particles at a low cost.
Single-core semiconductor nanoparticles, which contain a single semiconductor material along with an outer organic passivating (or capping) layer, may have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface.
One method to eliminate defects and dangling bonds is to grow a second inorganic material, having a wider band-gap and small lattice mismatch to that of the core material, epitaxially on the surface of the core particle to produce a “core-shell” particle. One example is ZnS grown on the surface of a CdSe core to provide a CdSe/ZnS nanoparticle.
Another approach is to prepare a core/multi-shell structure where the “electron-hole” pair is confined to a single shell layer such as the quantum dot-quantum well structure. One example of such core/shell/shell structured nanoparticle is CdS/HgS/CdS.
The coordination about the final inorganic surface atoms in core, core/shell or core/multi shell nanoparticle is generally incomplete, with highly reactive atoms that are not fully coordinated leaving “dangling bonds” on the surface of the particle, which may lead to particle agglomeration. To overcome this problem, the core, core/shell or core/multishell is passivated (capped) with protecting organic groups. The outermost layer (or capping agent) of organic material helps to inhibit particle aggregation and also further protects the nanoparticle from its surrounding chemical environment. It also provides chemical linkage to other inorganic, organic or biological material.
However, the organic passivating layer (or capping layer) coated on a surface of the core or the shell of the nanoparticle act as an insulating barrier layer, causing a decrease in electric conductivity of the nanoparticle, which limits actual industrial applications of the semiconductor nanoparticles to various electronic devices such as a light-emitting diode, a solar cell, a transistor, and the like. If the organic capping layer is removed, a luminous efficiency of the nanoparticles may decline due to effects of dangling bonds, surface defects, etc. on the surface of the nanoparticle, or optical and electrical properties may decline due to charge carriers trapped on the surface of the nanoparticle. Also, removing the organic capping layer causes aggregation or fusion of nanoparticles and destabilizes the nanoparticles.
Recently, Science 2009, 324, 1417-1420 reported a research about replacing organic ligands of nanoparticles with metal chalcogenide complexes (“MCC”). The MCC have an electrical charge, and like the organic ligands, they function as a capping agent to form a coating layer on the surface of the nanoparticles in a solution and stabilize the colloidal nanoparticles in the solution. However, the research uses a hydrazine solvent which is highly toxic and highly explosive when exposed to air, which makes it difficult to apply the method to industrial fabrication process.
Therefore, there has been a need for a safe and efficient fabrication method of nanoparticles with an inorganic capping layer. nanoparticle